The conditions which are responsible for or can alter in vitro transformation with specific emphasis on the requirements of human fetal cells will be studied. These will include the use of X-rays and ultraviolet combinations of carcinogens of different classes, the combination of in vivo - in vitro systems, and the effect of stimulators and inhibitors of mixed function oxidases on toxicity and transformation. The mechanisms of early events of carcinogen-cell interaction will be studied. Investigation of different proximate carcinogens and metabolic inhibitors is planned. In addition, experiments are planned to test whether neoplastic transformation has occurred as a result of direct induction by chemical carcinogens, as the result of selection or of viral activation. The testing of the tumorigenicity of cells exposed for different time intervals will continue.